Backstress governs transient postseismic creep in the continental lithospheric mantle

Sheng Fan, Thomas Breithaupt, Claudia A. Trepmann, David Wallis

Understanding the physical processes that control how the continental lithospheric mantle responds to sudden stress changes during the earthquake cycle is essential for interpreting postseismic deformation. For example, recent experiments have revealed that elastic interactions among dislocations generate backstresses that control transient creep of olivine, and the relevance of this mechanism can be confirmed by analysing the stress fields of dislocations in various tectonic settings. Therefore, we examine peridotites from the Finero Complex in the European Alps, which preserve microstructures formed during transient, earthquake-related deformation in exhumed continental lithospheric mantle. High-angular resolution electron backscatter diffraction reveals pronounced intragranular stress heterogeneity in olivine, with variations of several hundred megapascals over length scales on the order of 1–10 µm and correlated with elevated densities of geometrically necessary dislocations. These observations demonstrate that long-range dislocation interactions occur in naturally deformed olivine of the continental lithospheric mantle. Deformation-mechanism maps indicate that both dislocation-mediated deformation and diffusion creep contribute at steady state. We construct a model of the transient rheological behaviour of olivine including contributions from dislocation-mediated deformation, as well as diffusion creep, and elastically accommodated grain-boundary sliding (EAGBS) to explore the characteristics of such transients in contexts similar to those represented by the Finero peridotite. This analysis indicates that an earthquake-induced stress increase produces a complex transient evolution in mantle strength. Following rapid initial relaxation by EAGBS, dislocation-mediated deformation dominates subsequent stress relaxation; its strain rate initially increases by approximately three orders of magnitude, followed by progressive restrengthening as long-range elastic interactions build up. These results demonstrate that time-dependent microphysical models, rather than flow laws constructed for steady state alone, are required to describe the postseismic evolution of mantle viscosity.